The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago). As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events. The extinction event that closed the Triassic period has recently been more accurately dated, but as with most older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are uncertain by a few million years.

The Triassic climate was generally hot and dry, forming typicalred bedsandstonesandevaporites. There is no evidence ofglaciationat or near either pole; in fact, the polar regions were apparently moist andtemperate, a climate suitable for reptile-like creatures. Pangaea's large size limited the moderating effect of the global ocean; itscontinental climatewas highly seasonal, with very hot summers and cold winters. It probably had strong,cross-equatorialmonsoons

Life

Three categories of organisms can be distinguished in the Triassic record: holdovers from the Permian-Triassic extinction, new groups which flourished briefly, and other new groups which went on to dominate the Mesozoic world.

In marine environments, new modern types of corals appeared in the Early Triassic, forming small patches of reefs of modest extent compared to the great reef systems of Devonian times or modern reefs. The shelled cephalopods called ammonites recovered, diversifying from a single line that survived the Permian extinction. The fish fauna was remarkably uniform, reflecting the fact that very few families survived the Permian extinction. There were also many types of marine reptiles. These included the Sauropterygia, which featured pachypleurosaurs and nothosaurs (both common during the Middle Triassic, especially in the Tethys region), placodonts, and the first plesiosaurs; the first of the lizardlike Thalattosauria (askeptosaurs); and the highly successful ichthyosaurs, which appeared in Early Triassic seas and soon diversified, some eventually developing to huge size during the late Triassic.

Terrestrial fauna

Temnospondylamphibians were among those groups that survived the Permian-Triassic extinction, some lineages (e.g. Trematosaurs) flourishing briefly in the Early Triassic, while others (e.g. capitosaurs) remained successful throughout the whole period, or only came to prominence in the Late Triassic (e.g. plagiosaurs,metoposaurs). As for other amphibians, the first Lissamphibia, characterized by the first frogs, are known from the Early Triassic, but the group as a whole did not become common until the Jurassic, when the temnospondyls had become very rare.

Archosaurs were initially rarer than the therapsids which had dominated Permian terrestrial ecosystems, but they began to displace therapsids in the mid-Triassic. This "Triassic Takeover" may have contributed to the evolution of mammals by forcing the surviving therapsids and their mammaliform successors to live as small, mainly nocturnal insectivores; nocturnal life probably forced at least the mammaliforms to develop fur and higher metabolic rates

Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease inspeciation than by an increase in extinction.

Several explanations for this event have been suggested, but all have unanswered challenges:

§Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm.

§Asteroid impact, but no impact crater has been dated to coincide with the Triassic–Jurassic boundary (the impact responsible for the annularManicouagan Reservoir occurred about 12 million years before the extinction event).

The isotopic composition of fossil soils of Late Triassic and Early Jurassic show no evidence of any change in the CO2composition of the atmosphere. More recently however, some evidence has been retrieved from near the Triassic–Jurassic boundary suggesting that there was a rise in atmospheric CO2 and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could have been a combination of volcanic CO2 outgassing and catastrophic dissociation of gas hydrate. Gas hydrates have also been suggested as one possible cause of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian Period.

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